1. Field of the Invention
Embodiments of the invention generally relate to optical waveguide structures for propagating light signals in a single polarization and to such structures and associated devices for employment in radioactive environments.
2. Description of the Related Art
Many optical components, such as fiber interferometric sensors, which measure the phase change due to optical path length changes in fiber optic implementations of Michelson, Mach Zehnder, Fabry-Perot, or Sagnac optical interferometers, require use of single mode optical fiber and waveguide devices. Such fibers may form components (e.g., the interferometers themselves, fiber gratings and optical fiber couplers) for use with various optical transmission or measurement devices such as interferometric fiber optic gyroscopes (IFOG). While only the lowest order bound mode can propagate with conventional single mode fiber, this light being guided may consist of a pair of orthogonally polarized eigenmodes such that cross-coupling between polarizations can cause interference and phase noise that can degrade sensor sensitivity and performance.
Optical systems which exhibit polarization dependence thus may require use of polarization maintaining (PM) optical fibers to reduce polarization cross-coupling. The PM optical fibers maintain the polarization state of polarized light signals launched into the fiber as the signals propagate through the length of the fiber due to birefringence of the fibers. However, cross-coupling still occurs in many applications with the PM optical fibers especially when coiling and packaging long lengths of the fiber for example in an IFOG, which introduces significant bending and mechanical perturbations that, along with any temperature fluctuations, promote cross-coupling between polarizations.
While maintaining polarization reduces occurrence of cross-coupling, some fibers operate to remove or eliminate any cross-coupled or otherwise unwanted polarization states to promote single-polarization operation. Performance, design, expense and manufacturability shortcomings of prior single polarization or polarizing fibers preclude use of these fibers in many operations and/or applications longer than a few meters. Examples of single polarization fibers include fibers having elliptical-shaped cores or lossy cladding regions to promote a difference in attenuation between polarization modes that achieves extinction of one polarization mode over fiber lengths of a few meters or less. However, guided polarization mode attenuation also occurs as a side effect limiting applications to fiber lengths of a few meters or less, such as in a polarizer filter or pigtail, since longer lengths tend to produce unacceptable low intensity levels of even the guided polarization mode that is to be measured or otherwise used. Other single polarization fibers utilizing more conventional core/cladding designs rely on differences in fundamental mode cutoff wavelength between polarization modes for single polarization operation. These fibers typically operate over a narrow wavelength band that is highly sensitive to fiber length, typically several meters, and the amount of bending placed on the length of fiber. This sensitivity results in limited packaging flexibility in achieving a desired polarization extinction.
In an exemplary application, a Sagnac interferometer may form an IFOG sensor constructed with long lengths (e.g., multiple kilometers) of sensing fiber since sensitivity is proportional to the sensing fiber length. However, increases in length of the fiber amplify undesired polarization effects that impair sensor performance, which is limited by signal strength to phase noise (i.e., optical signal-to-noise ratio, OSNR) and is hence proportional to the amount of polarization cross-coupling. In addition, applications of the IFOG sensor include navigation systems employed in space and military operations where ionizing or nuclear radiation-induced attenuation (RIA) further contributes to signal loss and thus reduction in optical signal-to-noise ratio. Radiation resistant fibers include single mode designs that permit significant polarization cross-coupling. Problems associated with the RIA and/or the polarization cross-coupling thwart attaining critical performance requirements and prevent ability to maintain design OSNR for these IFOG sensors.
Therefore, there exists a need for improved methods and waveguides that propagate light signals in a single polarization. A further need exists for such single polarization waveguides with improved resistance to radiation-induced attenuation to enable devices such as an IFOG utilizing the waveguide, to be employed in radioactive environments.